1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related mobile device, and more particularly, to a method of controlling channel access for a mobile device in a wireless communication system and related mobile device using the method.
2. Description of the Prior Art
Cellular networks provide the infrastructure to support mobile phone devices. The network is organized in a series of cells to provide coverage to different areas. At the centre of each cell is a base station, which provides connections from the mobile devices back into the fixed telecommunications backhaul network to route traffic to its end destination.
The base station is unaware of a mobile device within its cell coverage until the mobile device makes contact with the base station. In a UMTS (Universal Mobile Telecommunications System) system, the mobile device, also known as user equipment (UE), makes initial contact with the base station over a channel called a Random Access Channel (RACH). However, the mobile device first needs to know the characteristics of the RACH for the cell that it is currently operating in. RACH characteristics, amongst other system information, are provided by the base station via a Broadcast Channel (BCH) which is transmitted to all mobile devices across the cell.
In addition, the UE can access the base station via more than one random access channels. By random access, there can be a number of UEs attempting to make simultaneous access attempts using the channel. A consequence of these simultaneous access attempts is collision, resulting in a reduction in the capacity of the channels. In order to reduce the collision, UMTS uses Access Service Classes (ASC) shown below to define how and how often a specific UE can use these RACHs.
ASC # i01234567Pi1P(N)s2 *s3 *s4 *s5 *s6 * P(N)s7 *P(N)P(N)P(N)P(N)P(N), where P(N) = 2−(N−1).
ASC is defined by an identifier i that defines a certain partition of the PRACH resources and may be numbered in the range 0≦i≦NumASC≦7 (i.e. the maximum number of ASCs is 8). In addition, ASC is associated with persistence value Pi. The persistence values Pi to be associated with each ASC shall be derived from the dynamic persistence level N=1, . . . , 8 and the persistence scaling factors si is broadcasted from base stations of UMTS system.
ASC#0 is assigned the highest priority and ASC#7 is assigned the lowest priority. ASC#0 shall be used to make emergency calls which has got more priority and more PRACH resources. The persistence value for P0 is always set to 1 and is associated with ASC#0. The persistence values for others are calculated from broadcasted signaling. These persistence values controls the RACH transmissions.
However, if a large number of mobile devices with ASC#0 setting the persistence value to 1 continuously try to make contact with the base station, e.g. make emergency calls, via the RACH, the RACH can become overloaded to the extent that messages cannot get through to the base station. This causes low probability of successful RACH transmission for a mobile device.